Snakes that snack on poison

They say you are what you eat. And that's especially true of Rhabdophis tigrinus — zoologists have discovered that this snake eats poisonous toads and keeps their venom for itself.

Rather than going to the trouble of making its own venom to use against predators, R. tigrinus, which is found in Asia, takes the venom from its prey and transports it to its own venom glands for storage and use.

The snakes eat a wide range of prey, often including toads that secrete defensive poisons called bufadienolides through their
skin. When fed a diet featuring these toads, the snakes' venom glands fill up with an almost chemically identical venom, report
Deborah Hutchinson of Old Dominion University in Norfold, Virginia, and her colleagues.

Snakes lacking toads in their diet do not gather the poison, the researchers add. Their findings are published in Proceedings of the National Academy of Sciences1.

Toxic treats

Many invertebrates, such as sea slugs, collect and store toxins from their plant food to make themselves unpalatable to predators.
A few species of poisonous frogs also get their toxins from insects in their diet. But examples of vertebrate predators using
venom from vertebrate prey are rare, and the only other species known to do it only stores venom temporarily.

The snakes store the poison in structures called nuchal glands on the back of their neck. When confronted by a hawk, their
main predators, they mount a defensive display, arching their neck to display the glands. If the bird bites or scratches the
snake, the venom is released.

Snakes with no toads, and therefore no toxin, in their diets do not make these displays, Hutchinson and her colleagues found.
"Those snakes just tend to run away," she says.

But if there is enough venom available, mothers can even provide their young with venom before they hatch, either by supplying
it in the yolk of their eggs, or by transferring it through the walls of their egg-laying canal later during development,
the researchers report.

Hutchinson and her team still don't know exactly how the snakes squirrel away the poison and transfer it to their venom glands
without digesting it. It is also unclear how the toxin is chemically modified — or which version is the more potent.